From the past, various types of soft type and hard type contact lenses have been provided with the object of correcting myopia, hyperopia, astigmatism, presbyopia or the like.
Meanwhile, this kind of contact lens is worn overlapping the surface of the cornea, which is a sensitive biological tissue. Therefore, in addition to optical characteristics adapted for the wearer, it is required for the contact lenses to suppress the sense of a foreign object when worn and to realize a wearing comfort.
Also, to realize a wearing comfort with reduced irritation on the cornea, when fitting a contact lens on each person, a radius of curvature corresponding to the cornea of the wearer is selected among a plurality of base curves set for the lens rear surface. For example, with many of the disposable type soft contact lenses on the market, there is a base curve setting standard of every 0.1 mm within a range of 8.5 mm to 9.0 mm.
However, there are individual differences in corneal surface shape, and with base curves of that kind of standard, there were cases when it was difficult to obtain a sufficiently satisfactory wearing comfort. Particularly with hard contact lenses, even if the shape difference between the lens rear surface and the corneal surface is small, the pressing sensation is strong, and it is easy for the wearing sensation to be a problem. Also, with soft contact lenses, if the difference between the lens rear surface and the corneal surface is big, there is a risk of the lens displacement volume on the cornea when worn becoming big, resulting in a decrease in vision.
Rather than simply selecting a base curve of the contact lens, to realize a high level of shape fitting of the contact lens rear surface on the corneal surface, PCT Japanese Translation Patent Publication No. JP-A-2004-534964 (Patent Document 1) proposes manufacturing a made-to-order contact lens for which the cornea shape is actually measured for each individual, and a shape corresponding to the actual measurement results is noted mathematically so as to be reflected in the lens rear surface shape.
However, when manufacturing the made-to-order contact lenses as noted in Patent Document 1, the contact lenses must be designed and produced using a lens rear surface shape for which the corneal surface shape for each lens user (wearer) is measured individually, and furthermore, the measurement results are faithfully reflected. Therefore, a vast amount of labor, time and cost is required, making it very difficult to put that to practical use.
In fact, it is difficult to represent actual measurements of the complex cornea shape using mathematical notation, and we at least have to say that the practicality is poor with the invention noted in Patent Document 1. Even if it is possible to mathematically represent the complex surface shape of the cornea, the mathematical notation is extremely complex, and when understanding the specific shape or doing processing, numerical value handling is difficult in regards to things such as to what level of precision processing should be done on which numerical values. Thus, it is difficult to reflect the mathematical notation to the lens inner surface shape to produce a contact lens, and the time, labor and technical burden required when producing the lens is extremely large. Also, when forming the surface shape represented by a complex mathematical notation on the actual lens rear surface, there is the problem that it is easy for the processing error to become large. In fact, even currently with approximately ten years elapsed since publication of Patent Document 1, contact lenses having the made-to-order lens rear surface like that noted in Patent Document 1 have not been provided to the market, so we can understand that it is difficult to put this to practical use.